1. Technical Field of the Invention
The invention relates to a method and system for converting non-printable colour values to printable colour values.
2. Description of Related Art
A method of this kind is known from the International Patent Application WO95/22866, which mentions the problem when input colours cannot be reproduced by an image reproduction apparatus such as, for example, a printing device. This is due to the fact that a source of image signals, such as, for example, a scanner, display or graphic page composition software program, has a different colour range, also referred to as the xe2x80x9cgamutxe2x80x9d, from an image reproduction apparatus such as a printing device or a display. In a printing device, the colour range to be reproduced is determined, inter alia, by the primary inks used, as in the case of an inkjet printer, or by the primary colour toners in the case of an electrophotographic printer.
This problem can generally be solved by compressing the entire input colour range that it fits in the reproducible output colour range. The disadvantage of this conventional technique is that all the input colour values, including the original values already situated in the output colour range, are changed.
Another conventional method converts only the colour values not situated in the output colour range to the closest colour values of the boundary of the output colour range, this process being known as clipping. For this purpose, a minimum Euclidean distance is calculated, in the colour space in which the colour values are defined, between an input colour and the closest output colour. This method is also used in the method described in the above-identified patent application.
A well-known disadvantage of clipping is that input colour values which are different from one another can be converted to one and the same output colour value. Another disadvantage of clipping is that two neighboring input colour values can be converted to two output colour values situated further apart. This occurs particularly if the boundary of the output colour range has a non-continuous configuration, the boundary in the colour space having sharply defined outwardly directed offshoots. Such offshoots usually correspond to the more intensively saturated primary colours as compared to mixed colours of lower saturation obtained by mixing the primary colours. Input colour values will then be converted to colour values corresponding to these offshoots rather than to the colour values situated between these offshoots, since the colour values situated further outside the input colour space are frequently the ones situated closest to these offshoots.
The method according to the invention overcomes these disadvantages. The present invention extends, in accordance with the selection rule, the closest part of the boundary by a part of the boundary situated in the colour space further away from the first colour co-ordinates, in dependence on the distance of the first colour co-ordinates from the boundary and establishes the second colour co-ordinates on the basis of the extended part of the boundary.
The reproduction system according to the invention includes a colour conversion unit adapted to generate the second colour value signal corresponding to the second colour co-ordinates, on the basis of colour co-ordinates which are situated in an extended part of the boundary of the predetermined colour range, which extended part is situated at a greater distance from the first colour co-ordinates, the extended part being determined in dependence of by the distance of the first colour co-ordinates from the boundary.
Instead of just taking for the conversion the part of the boundary situated closest to the first colour co-ordinates, consideration is now also given to parts of the boundary situated further away. In this way, the influence of any discontinuity in the boundary on establishing the second colour co-ordinates is reduced since a larger number of options is now offered. By taking into account the distance in the colour space between the first colour co-ordinates for conversion and the boundary, a gradual progression is obtained in the size of the extended part of the boundary so that there are less options for conversion for first colour co-ordinates situated close to the boundary than for colour co-ordinates situated further away.
Another embodiment of the method extends the boundary by selecting a part of the boundary which is situated between a first and second distance from the first colour co-ordinates defined in the colour space, wherein the first distance corresponds to the distance between the part of the boundary situated closest to the first colour co-ordinates, and the first colour co-ordinates, and the second distance is larger than the first distance. The extended part of the boundary can in this way be unambiguously determined while the colour co-ordinates corresponding thereto are always situated within a specific maximum distance from the first colour co-ordinates.
One advantageous embodiment of the method establishes the second distance by enlarging the first distance by a value equal to a constant factor times the first distance. The effect of this is that the closer the first colour co-ordinates are situated to the boundary so the extended part of the boundary for selection becomes smaller and finally coincides with the first colour co-ordinates. In this way, a continuous connection is obtained to the first colour co-ordinates which do come within the output colour space.
One embodiment of the method is determines a distance between two colour values in the colour space on the basis of a summation of quadratic differences between the corresponding colour co-ordinates for at least two colour parameters. This is equivalent to a Euclidean definition of distance and follows on the conventional definitions of distance in a colour space. In a colour space defined by colour parameters for lightness L, chroma C and hue H, a distance in a plane can be utilized by reference, for example, just to the parameters L and C, or in the space by reference to the parameters L, C and H.
A further embodiment of the method determines a distance by summating the quadratic differences weighted with different weighting factors for each colour parameter. By giving heavier weighting to one of the colour parameters, e.g. the hue, in the calculation of the distance, the shifts in hue H can be reduced.
A further embodiment of the method is obtained by establishing the second colour co-ordinates by determining an average of the selected colour values of the extended part of the boundary. By taking into account all the colour values situated on the boundary the influence of a non-continuous configuration of the boundary on the establishment of the second colour co-ordinates is reduced.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.